Motion sensor system

ABSTRACT

A system for providing grade crossing protection by sensing the presence of trains exceeding a preselected velocity in an approach track section defined by shunts across the rails. Detector circuits provide fail-safe protection from defects in the associated track section such as a broken rail or low ballast impedance between rails. A time-sharing self-test subsystem simulates an approaching high speed train by applying a fraction of a second test signal to the system every few seconds to monitor the operation of the overall system; and a low gain detector checks the gain of the system to assure that the train&#39;&#39;s speed is accurately measured.

United States Patent Peel 1 1 MOTION SENSOR SYSTEM [76] Inventor: Richard V. Peel, 62 3 Greenhom Dr., Diamond Bar, Calif. 91766 [22} Filed: Dec. 3, 1970 [21] Appl. No.: 94,955

Dec. 4, 1973 {5 7 ABSTRACT I A system for providing grade crossing protection by sensing the presence of trains exceeding a preselected I22] 246/l25l3r6211l6/l12)2 velocity in an approach track Section defined by l d 6 125 shunts across the rails. Detector circuits provide fail- 1 e 0 are l safe protection from defects in the associated track section such as a broken rail or low ballast impedance [56] References Clted between rails. A time-sharing self-test subsystem simu- UNITED STATES PATENTS lates an approaching high speed train by applying a 3,246,143 4/1966 Steele et al 246/128 fraction of a second test signal to the system every few 3,390,256 6/1963 ha t 6t 125 X seconds to monitor the operation of the overall sys- 3,610,92O 10/1971 Fnehnghans 246/128 and a 1 i detector checks the i f the system to assure that the trains speed is accurately measured.

14 Claims, 9 Drawing Figures "/2 \--APPR0ACII DIsTANcE\ 1 I4 1 A C 7 A c /6 SHUNT SHUNT '44s 4&

POWER MODULATOR AMPLIFIER 44A" W36 TRACK I -46 T g y SIMULATOR "42 5O OSCILLATOR 26 L38 113E2 RECTIFIER 26 AMPLIFIER FILTER -34 D 5/ I :52 I 53 I /54 55 MOTION a TRACK HIGH SIGNAL I ow GAIN LOW SIGNAL SELF-CHECK OCCUPIED DETECTOR DETECTOR DETECTOR DETECTOR F DETECTOR 59 TIMING GENERATOR PATENTED 4 7 snm 30F v 5 INVENTOR. RICHARD V. P EL ATTORNEY v GI PATENTEU UEE 41975 SHEET b 0F 5 mwnEOIQ TOE m0 mm E mohqmwzmw OF N: kojwm J INVENTOR. RICHARD V PEEL BY ATTORNEY 1 MOTION SENSOR SYSTEM BACKGROUND OF THE INVENTION This invention relates generally to systems which provide a warning of a train approaching a grade crossing; and more particularly to fail-safe systems which sense the approach of a train and activate grade crossing protection devices when the approach speed is greater than a preselected value or when a system malfunction occurs.

Prior systems, such as those described in US. Pat. Nos. 3,246,143 and 3,390,256, used data as to the distance and velocity of an approaching train to compute the estimated time of arrival of a train at a protected grade crossing, and to activate an associated crossing warning device a fixed time before the train arrived at the crossing. These systems, although quite effective and necessary in some applications, are relatively expensive, especially when fail-safe features are incorporated. A significant aspect of thesubject invention is the recognition that for many applications adequate grade crossing protection may be provided by a relatively simple motion sensor system which detects the motion of a train on a predefined approach track section if the trains velocity toward the crossing exceeds a selected value.

A major advantage of the motion sensing approach of the subject invention is that it allows the incorporation of fail-safe features which detect malfunctions in both the system and the associated track section by means which are simple, inexpensive to mechanize, and reliable in operation.

SUMMARY OF THE INVENTION It is therefore a general object of the subject invention to provide an improved railroad grade crossing protection system.

Another object is to provide a railroad grade crossing warning system which is efficient and reliable and which incorporates fail-safe protection against equipment malfunctions.

Yet another object is to, provide a railroad grade crossing warning system which protects against malfunctions not only within the system,.but against those in the associated track section as well.

Still another object is to provide a railroad grade crossing protection system incorporating a timesharing self-check capability, whereby the approach of a high speed train is simulated for a fraction of a second every few seconds to continually monitor the operational status of the overall system.

Briefly the subject invention comprises a transmitter unit which excites a track approach section with a constant current AC signal. The track section functions as a transmission line which is shorted by a train; and a receiver coupled to the track section responds to the change in voltage (ED), resulting from the approach of a train to determine its speed. If the train is approaching at a speed above a preselected value, the crossing protection equipment is activated by a motion sensor unit. If the track section is unoccupied or a train is stopped in the approach section, or if the train is moving away from the crossing, the crossing protection system will not be activated unless a malfunction has occurred within the motion system or the associated track section.

A broken rail or bondcauses the track impedance to increase thereby increasing the value of ED. A high signal detector circuit senses this increase in the voltage E and in response thereto activates the crossing protection equipment. If the ballast impedance between the rails decreases below a selected value, a low signal detector circuit indicates the occurrence of the defect by activating the crossing protection equipment. Means are provided for preventing the low signal detector unit from indicating a warning condition when a train is in the approach section so that the motion sensor unit may control the operation of the crossing protection equipment when the approach section is occupied.

A low gain detector continually monitors the gain of the system to insure that the trains speed is accurately measured. For this purpose an adjustable inductor is coupled in series with one rail and the transmitterreceiver unit. The inductor is adjusted such that a selected ratio is established between the impedance of the track and the impedance of the inductor. Hence a continuous component of the E voltage is the result of the impedance of the inductor, and this voltage component is used to insure that system gain variations do not cause measurement errors which exceed selected limits. If the system gain does decrease by more than the allowed percentage, the low gain detector unit activates the crossing protection system to indicate the malfunction,

A time-sharing self-check unit simulates the approach of a high speed train by modulating the transmitter output signal for a fraction of a second every few seconds, and includes means for monitoring a received signal resulting therefrom and for processing the received signal to indicate a warning condition if a malfunction exists within the system.

BRIEF DESCRIPTION OF THE DRAWINGS The novel features which are characteristic of the invention, both as to its organization and method of construction and operation, together with further pbjects and advantages thereof will be better understood from the following description taken in conjunction with the accompanying drawings in which the illustrative embodiments ofthe invention are disclosed and wherein likereference numerals indicate like or corresponding parts.

In the drawings:

FIG. 1 is a block and schematic diagram of a motion sensor system in accordance with the principles of the subject invention, installed in association with a track approach section for bidirectional control of a grade crossing;

FIG. 2 is a block diagram of a motion and self-check detector unit suitable for incorporation into the system of FIG. 1;

FIG. 3 is a block diagram of a low signal detector unit and a track occupied circuit suitable for incorporation into the system of FIG. 1;

FIG. 4 is a block diagram of the .high signal detector and low gain detector units of FIG. 1;

FIG. 5 is a schematic diagram of aportion of the motion and self-check detector unit of FIG. 2;

FIG. 6 is a schematic diagram of a portion of the low signal detector and track occupied units of FIG. 3;

FIG. 7 is a switching circuit diagram useful in explaining the invention;

FIG. 8 is a block diagram showing the installation of the system of the invention for unidirectional control of a grade crossing; and

FIG. 9 is a block diagram showing the installation of the invention for independent control of both approach directions to a grade crossing.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, the motion sensor system there shown is coupled to the associated track section so as to control crossing protection equipment suh as gates and/or signals, associated with a railroad crossing 10. The system of FIG. 1 will activate the crossing protection equipment whenever a train, within a section of track being monitored, approaches the crossing at a speed greater than a preselected speed (X mph) or when a malfunction occurs in the system.

In the operation of the subject invention, the approached length of track becomes an integral part of the sensor system, and this length is established as a function of the maximum train speed, a minimum warning time and the systems response time so that the crossing gates are closed in sufficient time to provide adequate protection of the crossing.

As shown in FIG. 1, the right-hand approach distance 12 is the track distance from a feed point 14 to a shunt 16. The left-hand approach distance is the correspond ing interval from feed point 14 to shunt 18. The left and right approach distances may be equal; however, they need not be, and if the train speed limit is different for trains moving in opposite directions, the approach distances will be correspondingly different on opposite sides of the crossing. The approach section 17 is that section of track between the feed point 14 and the appropriate shunt 16 or 18, depending on train direction. Shunts 16 and 18 are coupled between rails 20 and 22 and they may be hardwire, wide band AC devices (a capacitor) or narrow band AC devices (a sharply tuned series resonant circuit) depending upon other signals which are being transmitted through the rails.

The operation of the motion sensor system of the subject invention is based upon the change in impedance of the track as an approaching train shunts rails 20 and 22, thus shortening the effective length of the track section. The motion sensor system responds to the approaching motion of a train to activate the crossing protection equipment if the trains speed is above X mph, for example, 2 mph. When the train has cleared the crossing the motion sensor system will no longer sense aproaching motion and the crossing equipment will be inactivated.

If the train stops beforereaching the crossing or backs away from the crossing, the protection equipment will be inactivated as approaching motion is no longer sensed. If the train resumes forward motion toward the crossing, the protection equipment will again be activated and remain so until the train has cleared the crossing.

The motion sensor system may be used in conjunction with a short range track (island) circuit (not shown). The short range track circuit is not dependent upon train speed but causes the crossing protection equipment to be activated whenever a train enters the island zone, which may be to feet on both sides of its connections to the track. This short range track circuit will maintain the crossing protection equipment activated even though the train is stopped within the island zone or its moving within the zone at a speed too slow to cause activation of the motion sensor system.

The motionsensor system includes a transmitter section 24 and a receiver-processor section 26.

Transmitter 24 includes an oscillator 28 coupled through a modulator 30 to a power amplifier 32. Oscillator 28 provides a selected frequency output signal which signal may have a frequency within the range of 26 to 645 Hz, for example; and power amplifier 32 may include a feedback circuit to provide a constant output current for a given level of input signal applied thereto. Modulator 30, which may include a switchable resistor scaling network, varies the oscillator signal level ap plied to the power amplifier 32. A control signal V applied to the modulator 30 on a lead 34 controls the operation of the modulator.

The output signal from power amplifier 32 is coupled through a transformer 36 to isolating resistors 38 and 40. One end of resistor 38 is coupled on a lead 44A to rail 22 at feed point 14. The feed point 14 on rail 22 is coupled by a lead 44B to one input terminal of a band pass filter amplifier 42 of receiver-processor section 26. One end of resistor 40 is coupled to a track simulator unit 46; and to the other input terminal of filter amplifier 42. Track simulator unit 46 may include an adjustable inductor for the purpose of detecting low gain conditions as will be explained hereinafter, and its output is coupled to rail 20 at feed point 14 by a lead 48.

The impedance of the track section 17 between shunts 16 and 18 is a constant under normal conditions, and when a train enters the section its wheels shunt the track, changing the effective length of the section and therefore the effective impedance thereof. This change in impedance in turn causes a corresponding change in the voltage applied to the input terminals of band pass filter amplifier 42.

The output signal of the filter 42 is converted to a DC signal E, by a rectifier filter unit 50, and this signal E is applied to detector units 51 through 54 shown in FIG. 1. I

Motion and self-check detector 51 is shown in greater detail in FIG. 2 to which reference is now primarily directed. The distance voltage E is applied to a differentiator unit 60 whose function is to measure the rate of change of voltage E As a train approaches, it will cause the track impedance to decrease, in turn causing E to change in value proportionally to the track impedance. If the speed of an approaching train is at least X mph, a negative output signal is produced by differentiator 60 which exceeds the threshold level of an amplitude discriminator circuit 62. Discriminator circuit 62, which may be a Schmitt trigger circuit, when triggered activates relay driver 64 thereby causing a motion relay 66 to energize.

If there is no change in the value of voltage E as in the case of an unoccupied track or a train stopped in the approach, the differentiator will produce no output signal and the Schmitt trigger 62 will not fire. Thus the motion relay 68 will remain de-energized. If a train is moving away from the crossing, having passed through the crossing, or having stopped in the approach section is backing away from the crossing, differentiator 60 will produce a positive voltage. Since the amplitude discriminator 62 is designed to operate only on a negative voltage it will not trigger and the motion relay 66 will remain de-energized.

Motion relay 66 controls motion delayed relay circuit 68 which includes a relay held energized through a set of normally closed contacts (not shown) of motion relay 66. When relay 66 is activated, delayed motion relay 68 drops out after a preselected delay period of 2 seconds, for example. Drop out of motion delayed relay 68 causes a second motion delayed relay 70, which is normally energized, to drop out at the end of a second delay period, such as 3 seconds, for example, following the drop out of relay 68. Contacts of relay 70 control a circuit (to be described hereinafter) in a selfcheck time constant circuit 72 to trigger a Schmitt trigger circuit 74. Trigger circuit 74 controls a relay driver 76 which in turn causes a self-check and motion relay 78 to drop out. Contacts 80 of relay 78 (FIG. 7) are in series with the voltages to a crossing control relay 82 and when relay 78 drops out, it results in activation of the crossing protection equipment controlled by relay The voltage E is also applied to. low signal detector (LSD) unit 54 which is shown in greater detail in FIG. 3. When E falls below a preselected level an amplitude discriminator 84, which may be a Schmitt trigger device, causes relay driver 86 to de-energize low signal detector relay 88. When low signal detector relay 88 drops out, it removes power from a low signal detector delayed relay circuit 90, which drops out after a delay of 4 to 5 seconds, for example.

Contacts 89 (FIG. 7) of relay 90 are in series with the power path of crossing control relay 82 and this path is interrupted when relay 90 is de-energized. This is a fail-safe feature which closes the crossing gates if an abnormal condition such as low ballast impedance develops across the rails of section 17 (FIG. 1).

The threshold level of amplitude discriminator 84 may be set such that the low signal detector relay 88 will drop out when the train has moved into the approach section by approximately 30 percent of the approached distance, for example. Track occupied detector 55 is provided to prevent the indication of a low signal. condition when a train is in the approach section. As shown in FIG.;3, detector 55 includes a latching circuit 92 coupled to a track occupied relay 94. The latching circuit 92 is enabled by motion delayed relay 68 (FIG. 2) and is released by the low signal detector delayed relay 90.

The operation of the latching circuit 92 and track occupied relay 94 may be explained by reference to FIG. 6 which shows these circuits in greater detail. As shown in FIG. 6, when motion delayedrelay 68 (FIG. 2) drops out, it applies a power path through the contacts of energized low signal detector delayed relay 90 and the contacts of delayed motion relay 68 to track occupied relay 94 which will pick up immediately. A set of contacts 96 (FIG. 7) on the track occupied relay 94 provides a power path to the crossing control relay 82 parallel to the path provided by the low signal detector delayed relay 90 contacts 89 of FIG. 7. This just described feature is to prevent activating the crossing protection equipment due to a low signal condition if a train is actually in the approach section since the low signal detection delayed relay 90 drops out as each train moves through the approach section. A second set of contacts 97 on the track occupied relay 94 provide a lockup path for this relay so that when relay 90 drops out power will still be available to relay 94 even when relay 68 has energized and removed the original energizing path to relay 94.

When relays 88m 90 become energized due to the train leaving the approach section, the lockup configuration for relay 94 is broken and this relay then drops out. For example, when the train has moved across 70 percent of the approach distance, the E signal will be high enough to pick up the low signal detector relay 88 thereby applying power to the delayed relay 90 which will pick up almost immediately. This causes the track occupied relay 94 to drop out after a preset delay of 4 to 5 seconds,,for example, thereby returning the entire system back to the unoccupied track condition.

A mechanization of high signal detector 52 and low gain detector 53 is shown in FIG. 4. The function of the high signal detector 52 .is to recognize an open track condition such as caused by a broken rail, a broken bond, or a break in the termination which results in an increased impedance and an abnormally high voltage E As shown in FIG. 4 voltage E is coupled through a resistor-divider network 100, is modulated by a chopper 102 and the value of the chopped signal is sensed by an amplitude discriminator 104 which controls a high signal and low gain relay 106. When the signal to discriminator104 increases above a preselected value, relay 106-drops out and contacts 108 thereof (FIG. 7) interrupt the power path to crossing control relay 82.

Low gain detector 53 comprises an input resistor network 109, a chopper 111 and an amplitude discriminator 110. When the system gain decreases below a selected valve, discriminator 110 disenables chopper 102 resulting in relay 106 dropping out and thereby indicating a malfunction condition.'The simulated track unit 46 (FIG. 1) is adjusted to provide an adequate E voltage to keep detector 53 from indicating a warning condition under normal gain conditions even when a train is in the approach section.

Motion and self-check detector 51 contains circuitry for periodically performing-a system self-test except during the time period when the motion of an approaching train has been sensed. For example, the selfcheck interval may be 0.6 seconds at the beginning of The self-check circuitry includes units 72, 74, 76 and 78 (FIG. 2) and a portion thereof is shown in greater detail in FIG. '5.

As shown in FIG. 5, the timing generator 59 (FIG. 1) energizes scale factor relay 112 for a 0.6 second interval occurringevery 6 seconds. A modulation relay 114 is energized through contacts 116 of relay 112. When relay 112 picks up, power is removed from modulation relay 114. After a 0.2 second delay, for example, to allow other self-check functions to stabilize before modulation occurs, relay 114 drops out.

When relay 114 drops out, a set of contacts thereof (not shown) changes scale factor values in modulation unit 30 (FIG. 1) and thereby reduces the current applied to the track section for approximately 0.4 seconds. This will cause the voltage E to decrease which will be interpreted by the differentiator 60 as a high speed train approaching the crossing. The output of differentiator 60 during this first test cycle will operate discriminator 62causing motion relay 66 to energize for 0.4 seconds.

Prior to the self-test, a capacitor 118 is charged to V through the contacts 123 of modulation relay 114 and contacts 120 of motion relay 66. The other side of capacitor 118 is coupled to ground through contacts 121 of relay 114. When the modulation relay 114 drops out, a negative charge is transferred through a diode 122 and a resistor 126 to a capacitor 124. Between selfchecks, capacitor 124 discharges through resistor 128. The charge on capacitor 124, modulated by chopper 125, is sufficient to operate Schmitt trigger 74 (FIG. 2) and keep relay 78 picked up between self-test cycles. However, if for some reason the self-check voltage fails to occur for two cycles, the charge on capacitor 124 is depleted and relay 78 (FIG. 2) drops out, thus providing a fail-safe condition.

The circuit indicated generally by reference numeral 130 is provided to maintain the accuracy of the just described self-check operation during the time when a train is in the approach section but before either or both motion delay relays 68 and 70 dro out.

Between self-check periods a capacitor 132 charges through a diode 134 and contacts 136 of relay 70 (FIG. 2) to the negative potential (an approaching train) of the output of differentiator 60. When scale factor relay 112 is energized during self-check the charge on capacitor 132 is applied through a resistor 138 to the input of an operational amplifier 61 (part of differentiator 60). This applied charge is equal and opposite to the signal E applied to-the input of differentiator 60 causing the output signal level to go to zero. As a result of this feedback action, when modulation occurs 0.2 seconds later, the differentiator is ready to accept the selfcheck voltage described previously.

For the case of a receding train, between self-check periods a capacitor 133 is charged through a diode 135 and contacts 137 to the positive potential of the output of differentiator 60. When relay 112 is energized during self-check, the charge on capacitor 133 is applied through a resistor 139 to the input of operational amplifier 61. This applied charge is equal and opposite to the signal E applied at the input of differentiator 60 causing the output signal level to go to zero, thereby insuring correct self-check operation for the case of a departing train.

The modulation relay 114 is interlocked through contacts 140 and 142 of motion delayed relays 68 and 70, respectively. As a result of this interlocking feature relay 1 14 remains energized when a train-is approaching so that the self-check feature is discontinued.

Contacts 144 (FIG. of motion delayed relay 70 couple capacitor 124 to ground when relay 70 drops out in response to the sensing of an approaching train. This causes capacitor 124 to discharge and results in relay 78 (FIG. 2) dropping out.

FIG. 8 illustrates the connection of a system for unidirectionally controlling the approach to a crossing. Insulated joints I50 and 152 isolate section 17 from the portion of the track to the left thereof. Operationally the system of FIG. 8 performs in a similar manner to that described previously for the arrangement of FIG. 1.

FIG. 9 illustrates the connections for independently controlling the approach section from both directions.

TI-Ie right-hand track section 17' is isolated from the magnitude of the rate of change of the voltage E is I used to sense the presence of a train approaching the crossing at a speed above a preselected value, and crossing protection equipment is activated in response thereto. The voltage E is also utilized to provide high and low signal and low gain fail-safe protection. A time shared self-test is performed to continually monitor the systems operability.

Thus there has been described a novel and improved motion sensor system which is highly reliable and economical to manufacture, install and maintain.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to preferred embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated and in their operation may be made by those skilled in the art, without departing from the spirit of the invention; therefore, it is intended that the invention be limited only as indicated by the scope of the following claims.

What is claimed is: 1. An apparatus for detecting an approaching train within a track section between a feed point and at least one low impedance connection across the rails, said apparatus comprising:

transmitter means coupled to the rails at the feed point'for applying a current to the track section;

receiver means coupled to the track section for producing a first signal representative of the impedance of said track section;

differentiator means, coupled to said receiver means,

for producing a second signal representative of the rate of change of said first signal; motion detector means, coupled to said differentiator means, for producing a warning signal when said second signal is of a selected polarity and exceeds a preselected magnitude, whereby the presence of a train'in the section, approaching the feed point at a speed greater than a preselected speed produces a warning. signal;

low signal detector means coupled to said receiver means for producing a warning signal when said first signal is less than a preselected value, whereby a defect in said track section which causes an abnormally low impedance therein produces a warning signal; and

track-occupied means, coupled to said low signal detector means and to said motion detector means, for inhibiting said low signal detector means from producing a warning signal during-the time period that a warning signal is produced by said motion detector means and for a preselected time period thereafter.

2. An apparatus for detecting an approaching train within a track section between a feed point and at least one low impedance connection across the rails, said apparatus comprising:

transmitter means coupled to the rails at the feed point for applying a current to the track section;

receiver means coupled to the track section for producing .a first signal representative of the impedance of said track section;

differentiator means, coupled to said receiver mean for producing a second signal representative of the rate of change of said first signal; motion detector means, coupled to said differentiator means, for producing a warning signal when said second signal is of a selected polarity and exceeds a preselected magnitude, whereby the presence of a train in the section, approaching the feed point at a speed greater than a preselected speed produces a warning signal; and

low gain detector means, coupled to said receiver means, for providing a warning signal when said first signal is less than a preselected value; said low gain detector means including an impedance device coupled between said transmitter means and one of the rails, and between said one rail and said receiver means.

3. The apparatus of claim 2 further comprising high signal detector means coupled to said receiver means for providing a warning signal when said first signal exceeds a preselected value.

4. The apparatus of claim 2 wherein said transmitter means provides an alternating current and said impedance device is an adjustable inductor.

5. The apparatus of claim 2 further comprising selfcheck means for providing a warning signal upon the occurrence of a malfunction in said apparatus; said self-check means including means coupled to said transmitter means, for periodically modulating said current, means coupled to said differentiator means, for sensing the periodic increase in the value of said second signal resulting from said modulation, and means for providing a warningsignal if said periodic increase in said second signal is not sensed for a preselected time.

6. The apparatus of claim 2 further comprising low signal detector means coupled to said receiver means for producing a warning signal when said first signal is less than a preselected value, whereby a defect in said track section which causes an abnormally low impedance therein produces a warning signal.

7. The apparatus of claim 2 further comprising low signal detector means coupled to said receiver means for producing a warning signal when said first signal is less than a preselected value, whereby a defect in said track section which causes an abnormally low impedanc'e therein produces a warning signal; and

high signal detector means coupled to said receiver means for providing a warning signal when said first signal exceeds a second preselected value.

8. The apparatus of claim 2 further'comprising low signal detector means coupled to said receiver means for producing a warning signal when said first signal is less than a preselected value, whereby a defect in said track section which causes an abnormally low impedance therein produces a warning signal; andself-check means for providing a warning signal upon the occurrence of a malfunctionin said apparatus; said self-check means including means coupled to said transmitter means, for periodically modulating said current, means coupled to said differentiator means, for sensing the periodic increase in the value of said second signal resulting from said modulation, and means for providing a warning signal within a track section between a feed point and at least 5 one low impedance connection across the rails, said apparatus comprising:

transmitter means coupled to the rails at the feed point for applying a current to the track section;

receiver means coupled to the track section for producing a first signal representative of the impedance of said track section; v

differentiator means, coupled to said receiver means, for producing a second signal representative of the rate of change of said first signal;

motion detector means, coupled to said differentiator means, for producing a warning signal when said second signal is of a selected polarity and exceeds a preselected magnitude, whereby the presence of a train in the section, approaching the feed point at a speed greater than a preselected speed produces a warning signal;

high signal detector means coupled to said receiver means for providing a warning signal when said first signal exceeds a preselected value;

low gain detector means, coupled to said receiver means, for providing a warning signal when said first signalis less than a second preselected value; said low gain detector means including an impedance device coupled between said transmitter means and one of the rails, and between said one rail and said receiver means; and

self-check means for providing a warning signal upon the occurrence of a malfunction in said apparatus; said self-check means including means coupled to said transmitter means, for periodically modulating said current, means coupled to said differentiator means, for sensing the periodic increase in the value of said second signal resulting from said modulation, and means for providing a warning signal if said periodic increase in said second signal is not sensed for a preselected time.

10. An apparatus for detecting an approaching train within a track section between a feed point and at least one low impedance connection across the rails, said apparatus'comprising:

transmitter means coupled to the rails at the feed point for applying a current to the track section;

receiver means coupled to the track section for producing a first'signal representative of the impedance of said track section;

differentiator means, coupled to said receiver means, for producing a second signal representative of the rate of change of said first signal;

motion detector means, coupled to said differentiator means, for producing a warning signal when said second signal is of a selected polarity and exceeds a preselected magnitude, whereby the presence of a train in the section, approaching the feed point at a speed greater than a preselected speed produces a warning signal;

low signal detector means coupled to said receiver means for producing a warning signal when said first signalis less than a preselected value;

high signal detector means coupled to said receiver means for providing a warning signal when said first signal exceeds a second preselected value;

low gain detector means, coupled to said receiver means, for providing a warning signal when said first signal is less than a third preselected value; said low gain detector means including an impedance device coupled between said transmitter means and one of the rails, and between said one rail and said receiver means; and

self-check means for providing a warning signal upon the occurrence of a malfunction in said apparatus; said self-check means including means coupled to said transmitter means, for periodically modulating said current, means coupled to said differentiator means, for sensing the periodic increase in the value of said second signal resulting from said modulation, and means for providing a warning signal if said periodic increase in said second signal is not sensed for a preselected time.

11. The apparatus of claim further comprising track-occupied means, coupled to said low signal detector means and to said motion detector means, for inhibiting said low signal detector means from producing a warning signal during the time period that a warning signal is produced by said motion detector means and for a preselected time period thereafter.

12. The apparatus of claim 10 wherein said selfcheck means further includes means for adjusting the scale factor of said sensing means during the modulation periods to compensate for the component of the second signal due to the motion of a train within said track section.

13. The apparatus of claim 12 further comprising means for inhibiting said self-check means during the time a warning signal is produced by said motion detector means.

14. The apparatus of claim 13 wherein said differentiator means includes a capacitor coupled to an amplifier, and said means for adjusting the scale factor includes an energy storage network normally coupled to the output of said amplifier, and means for switching said storage network from the output to the input of said amplifier shortly before and during said modulation periods. I

i IO! 

1. An apparatus for detecting an approaching train within a track section between a feed point and at least one low impedance connection across the rails, said apparatus comprising: transmitter means coupled to the rails at the feed point for applying a current to the track section; receiver means coupled to the track section for producing a first signal representative of the impedance of said track section; differentiator means, coupled to said receiver means, for producing a second signal representative of the rate of change of said first signal; motion detector means, coupled to said differentiator means, for producing a warning signal when said second signal is of a selected polarity and exceeds a preselected magnitude, whereby the presence of a train in the section, approaching the feed point at a speed greater than a preselected speed produces a warning signal; low signal detector means coupled to said receiver means for producing a warning signal when said first signal is less than a preselected value, whereby a defect in said track section which causes an abnormally low impedance therein produces a warning signal; and track-occupied means, coupled to said low signal detector means and to said motion detector means, for inhibiting said low signal deTector means from producing a warning signal during the time period that a warning signal is produced by said motion detector means and for a preselected time period thereafter.
 2. An apparatus for detecting an approaching train within a track section between a feed point and at least one low impedance connection across the rails, said apparatus comprising: transmitter means coupled to the rails at the feed point for applying a current to the track section; receiver means coupled to the track section for producing a first signal representative of the impedance of said track section; differentiator means, coupled to said receiver means, for producing a second signal representative of the rate of change of said first signal; motion detector means, coupled to said differentiator means, for producing a warning signal when said second signal is of a selected polarity and exceeds a preselected magnitude, whereby the presence of a train in the section, approaching the feed point at a speed greater than a preselected speed produces a warning signal; and low gain detector means, coupled to said receiver means, for providing a warning signal when said first signal is less than a preselected value; said low gain detector means including an impedance device coupled between said transmitter means and one of the rails, and between said one rail and said receiver means.
 3. The apparatus of claim 2 further comprising high signal detector means coupled to said receiver means for providing a warning signal when said first signal exceeds a preselected value.
 4. The apparatus of claim 2 wherein said transmitter means provides an alternating current and said impedance device is an adjustable inductor.
 5. The apparatus of claim 2 further comprising self-check means for providing a warning signal upon the occurrence of a malfunction in said apparatus; said self-check means including means coupled to said transmitter means, for periodically modulating said current, means coupled to said differentiator means, for sensing the periodic increase in the value of said second signal resulting from said modulation, and means for providing a warning signal if said periodic increase in said second signal is not sensed for a preselected time.
 6. The apparatus of claim 2 further comprising low signal detector means coupled to said receiver means for producing a warning signal when said first signal is less than a preselected value, whereby a defect in said track section which causes an abnormally low impedance therein produces a warning signal.
 7. The apparatus of claim 2 further comprising low signal detector means coupled to said receiver means for producing a warning signal when said first signal is less than a preselected value, whereby a defect in said track section which causes an abnormally low impedance therein produces a warning signal; and high signal detector means coupled to said receiver means for providing a warning signal when said first signal exceeds a second preselected value.
 8. The apparatus of claim 2 further comprising low signal detector means coupled to said receiver means for producing a warning signal when said first signal is less than a preselected value, whereby a defect in said track section which causes an abnormally low impedance therein produces a warning signal; and self-check means for providing a warning signal upon the occurrence of a malfunction in said apparatus; said self-check means including means coupled to said transmitter means, for periodically modulating said current, means coupled to said differentiator means, for sensing the periodic increase in the value of said second signal resulting from said modulation, and means for providing a warning signal if said periodic increase in said second signal is not sensed for a preselected time.
 9. An apparatus for detecting an approaching train within a track section between a feed point and at least one low impedance connection across the rails, said apparatus compRising: transmitter means coupled to the rails at the feed point for applying a current to the track section; receiver means coupled to the track section for producing a first signal representative of the impedance of said track section; differentiator means, coupled to said receiver means, for producing a second signal representative of the rate of change of said first signal; motion detector means, coupled to said differentiator means, for producing a warning signal when said second signal is of a selected polarity and exceeds a preselected magnitude, whereby the presence of a train in the section, approaching the feed point at a speed greater than a preselected speed produces a warning signal; high signal detector means coupled to said receiver means for providing a warning signal when said first signal exceeds a preselected value; low gain detector means, coupled to said receiver means, for providing a warning signal when said first signal is less than a second preselected value; said low gain detector means including an impedance device coupled between said transmitter means and one of the rails, and between said one rail and said receiver means; and self-check means for providing a warning signal upon the occurrence of a malfunction in said apparatus; said self-check means including means coupled to said transmitter means, for periodically modulating said current, means coupled to said differentiator means, for sensing the periodic increase in the value of said second signal resulting from said modulation, and means for providing a warning signal if said periodic increase in said second signal is not sensed for a preselected time.
 10. An apparatus for detecting an approaching train within a track section between a feed point and at least one low impedance connection across the rails, said apparatus comprising: transmitter means coupled to the rails at the feed point for applying a current to the track section; receiver means coupled to the track section for producing a first signal representative of the impedance of said track section; differentiator means, coupled to said receiver means, for producing a second signal representative of the rate of change of said first signal; motion detector means, coupled to said differentiator means, for producing a warning signal when said second signal is of a selected polarity and exceeds a preselected magnitude, whereby the presence of a train in the section, approaching the feed point at a speed greater than a preselected speed produces a warning signal; low signal detector means coupled to said receiver means for producing a warning signal when said first signal is less than a preselected value; high signal detector means coupled to said receiver means for providing a warning signal when said first signal exceeds a second preselected value; low gain detector means, coupled to said receiver means, for providing a warning signal when said first signal is less than a third preselected value; said low gain detector means including an impedance device coupled between said transmitter means and one of the rails, and between said one rail and said receiver means; and self-check means for providing a warning signal upon the occurrence of a malfunction in said apparatus; said self-check means including means coupled to said transmitter means, for periodically modulating said current, means coupled to said differentiator means, for sensing the periodic increase in the value of said second signal resulting from said modulation, and means for providing a warning signal if said periodic increase in said second signal is not sensed for a preselected time.
 11. The apparatus of claim 10 further comprising track-occupied means, coupled to said low signal detector means and to said motion detector means, for inhibiting said low signal detector means from producing a warning signal during the time period that a warning signal is produced by said motion detector means and for a preSelected time period thereafter.
 12. The apparatus of claim 10 wherein said self-check means further includes means for adjusting the scale factor of said sensing means during the modulation periods to compensate for the component of the second signal due to the motion of a train within said track section.
 13. The apparatus of claim 12 further comprising means for inhibiting said self-check means during the time a warning signal is produced by said motion detector means.
 14. The apparatus of claim 13 wherein said differentiator means includes a capacitor coupled to an amplifier, and said means for adjusting the scale factor includes an energy storage network normally coupled to the output of said amplifier, and means for switching said storage network from the output to the input of said amplifier shortly before and during said modulation periods. 